The present invention generally relates to true-time-delay phase modules, and more particularly, to a method, systems and apparatus for providing true-time-delay signals using optical inputs.
The increasing demand on bandwidth and reliability of airborne communication networks have stimulated the replacement of mechanically scanned antennas by phased array antennas allowing independent electronic control of antenna elements increasing the flexibility and the speed of beam forming. In phased array antennas, the phase and amplitude of each radiating element are traditionally controlled through switching the length of electrical delays feeding the antenna elements.
The mechanism of phased-array antennas employing electronically driven antenna elements with individually controllable phase-shift can be described as follows. The wavefront direction of the total radiated carrier wave is controlled through continuously and progressively varying the phase shift of each radiating element, achieving a continuous steering of the antenna. For a linear array radiating elements with individual phase control, the far field pattern along the direction of "PHgr" can be expressed by equation (1)                               E          ⁡                      (                          Φ              ,              t                        )                          =                              ∑                          n              =              0                        N                    ⁢                      xe2x80x83                    ⁢                                    A              n                        ⁢                          exp              ⁡                              (                                                      ⅈω                    m                                    ⁢                  t                                )                                      ⁢                          exp              ⁡                              [                                  ⅈ                  ⁡                                      (                                                                  ψ                        n                                            +                                                                        nk                          m                                                ⁢                        Λ                        ⁢                                                  xe2x80x83                                                ⁢                        sin                        ⁢                                                  xe2x80x83                                                ⁢                        Φ                                                              )                                                  ]                                                                        (        1        )            
where An is pattern of the individual element, xcfx89m is the microwave frequency, km=xcfx89m/c is the wave vector, "psgr"n is the phase shift, xcex9 is the distance between radiating elements and "PHgr" is the direction angle of array beam relative to array normal. The dependence of the array factor on the relative phase shows that the orientation of the maximum radiation can be controlled by the phase excitation between the array elements. Therefore, by varying the progressive phase excitation, the beam can be oriented in any direction. For continuously scanning, phase shifters are used to continuously vary the progressive phase. For example, to point the beam at an angle "PHgr"0, "psgr"n is set to the following value,
"psgr"n=xe2x88x92nkmxcex9 sin "PHgr"0xe2x80x83xe2x80x83(2)
Differentiating Equation (2), results in                               ΔΦ          =                                    -              tan                        ⁢                          xe2x80x83                        ⁢                                          Φ                0                            ⁡                              (                                                      Δω                    m                                                        ω                    m                                                  )                                      ⁢                          xe2x80x83                        ⁢                          (              rad              )                                      ,                            (        3        )            
It is clear that for a fixed set of "psgr"n""s if the microwave frequency is changed by an amount xcex94xcfx89m, the radiated beam will drift by an amount xcex94"PHgr"0. This effect increases dramatically as "PHgr"0 increases. This phenomenon is the so-called xe2x80x9cbeam squintxe2x80x9d, which leads to an undesirable drop of the antenna gain in the "PHgr"0 direction.
For wideband operation, it is necessary to implement optical true-time-delay steering technique such that the far field pattern is independent of the microwave frequency. In the approach of optical true-time-delay, the path difference between two radiators is compensated by lengthening the microwave feed to the radiating element with a shorter path to the microwave phase-front. Specifically, the microwave exciting the (n+1)th antenna element is made to propagate through an additional delay line of length Dn=nL("PHgr"0). The length of this delay line is designed to provide a time delay
tn("PHgr"0)=(nxcex94 sin "PHgr"0)/cxe2x80x83xe2x80x83(4)
for the (n+1)th delay element. For all frequencies xcfx89m, "psgr"n, is given by
"psgr"n=xe2x88x92xcfx89mtn("PHgr"n)xe2x80x83xe2x80x83(5)
With such a delay set-up, when the phase term nkmxcex94 sin "PHgr" inside Eq. (1) is changed due to frequency xe2x80x9choppingxe2x80x9d, the phase term "psgr"n will change accordingly to compensate for the change such that the sum of the two remains unchanged. Thus, constructive interference can be obtained in the direction "PHgr"0 at all frequencies. In other words, the elemental vector summation in the receiving mode or in the transmit mode is independent of frequency, which is crucial for ultra wide band operation for future PAAs.
Conventional phased-array antenna technologies have demonstrated good performance characteristics with limited practicality for commercialization. One conventional system includes utilizing microstrip reflecting array antennas with mechanical phasing for providing phased-array antennas. Unfortunately, mechanical phased microstrip antennas utilize expensive miniaturized motors for providing beam steering through mechanical rotation of each antenna element.
Another conventional system deploys using a fiber grating prism. In fiber Bragg grating prism technology, high performance reflection gratings can be easily fabricated in ultralow-loss optical fibers. However, this configuration requires very expensive fast wavelength tunable laser diodes.
A third conventional method uses thermo-optically switched silica-based waveguide circuits. The thermo-optically-switched silica-based waveguide circuits offers excellent delay time control in a compact structure where the length of waveguide is defined by photolithography. However, the cost associated with using wavelength tunable laser diodes and/or 2xc3x972 thermo-optic switches make commercialization impractical.
Advancements in conventional phased array systems have limited commercialization due to increased system complexity, employing very expensive devices, and requiring extremely difficult fabrication processes.
In accordance with teachings of the present disclosure, a method, system and apparatus are described for providing phase delayed signals using optical inputs.
In accordance with one aspect of the present invention a delay module is disclosed. The delay module includes a waveguide operable to transmit optical signals and at least one diffraction element positioned along the waveguide and operable to provide a true time delay.
In accordance with another aspect of the present invention, a system for providing phase delayed signals is disclosed. The system includes at least one optical source emitting an optical signal having a predetermined wavelength, an optical waveguide optically coupled to the at least one optical source, a diffraction element optically coupled to the waveguide, the diffraction element positioned along the waveguide at a predetermined distance.
In accordance with another aspect of the present invention, a method for fabricating a delay module is disclosed. The method includes providing a substrate having a waveguide, patterning a diffraction element in relation to the waveguide, and etching the patterned diffraction element to provide the delay module.
In accordance with another aspect of the present invention a system for providing a true time delays is disclosed. The system includes at least one optical source operable to provide a modulated signal having a predetermined wavelength, a waveguide operably coupled to the optical source, the waveguide operable to propagate the signal, plural diffraction gratings positioned at predetermined distance relative to one another and operable to diffract a portion of the modulated signal. The system includes a photodetector operably associated with one of the plural gratings for detecting the diffracted portion of the modulated signal.